The invention relates to an electrophotographic image forming apparatus and an image forming unit.
Recently, an image forming method by a digital system becomes main stream in the field of electrophotographic image formation accompanied with progress in the digital technology. In the image forming method by the digital system, it is demanded sometimes to realize a small dot image of one pixel such as 1,200 dpi, number of the dot per 1 inch or 2.54 cm. Therefore, a high quality image forming technology capable of precisely reproducing such the small dot is required. Demands for miniaturization, rising in the image resolution, and full color printing in copy machines particularly rise recently. In the case of printer, demand for stably printing images having the image quality equal to images formed by ordinal printing process rises also. Accordingly, higher image forming technology is required for stably forming a high quality toner image.
Investigations on the developing means and the electrophotographic photoreceptor have been carried out for obtaining the high quality image.
Developing means in the electrophotographic technology can be roughly classified into a dry developing means and a wet developing means.
The dry developing means is a method using a toner in a powder state and is further classified into a dry double-component type using a developer containing a toner, a carrier and another additive and a dry single-component type using a developer containing a toner and another additive other than carrier.
On the other hand, the wet developing means is a method using a liquid developer composed of a carrier liquid and a toner dispersed in the carrier liquid. The wet developing method has merits comparing with the dry developing method that the diameter of the toner is smaller and the transparency of the toner is higher than those of the developer for the dry developing method. Therefore, the wet developing method has peculiarities that a high quality toner image can be obtained either by analogical or digital system and also in monochromatic or color image formation, and further an image forming apparatus capable of saving energy can be obtained.
An image forming apparatus is proposed, cf. Patent Document 1 for example, in which a toner image formed on an electrophotographic photoreceptor is transferred onto a recording medium by applying bias voltage for transferring applied through a transfer roller contacting to the image carrying member by a bias voltage applying means, and an insulating layer having a volume resistivity of not less than 1×107 Ωcm and a harness lower than that of the surface of the image carrying member is provided on the surface of the transfer roller, and the bias voltage applying means has a function of applying removing bias voltage for transferring the toner adhering on the transfer roller onto the image carrier (for example, refer Patent Document “Tokkai 2004-94037”.
Furthermore, investigations about the hardness of the electrophotographic photoreceptor and that of the intermediate transfer member have been carried out for preventing locally lacking of the image transfer. Regarding such the point, it has been proposed to make the dynamic hardness of the surface of the electrophotographic photoreceptor to higher than that of the intermediate transfer member; cf. Patent Document “Tokkai 2003-149950” for example.
An electrophotographic photoreceptor containing particles in the surface layer thereof is proposed for obtaining a lot of high quality toner image; cf. Patent Document “Tokkai 2005-43623” for example. However, further improvement is demanded because white lacking in a solid image, lacking inside a character image, occurrence of black spots and fogging, lowering-in the image density and the image sharpness in the course of printing many prints are caused so that the desired high quality image cannot be stably obtained when an electrophotographic photoreceptor, hereinafter referred to as simply photoreceptor, according to such the proposals is applied for an photoelectric image forming apparatus, hereinafter sometimes referred to as simply image forming apparatus.